You have to accept those kinds of harms. Otherwise, I can spray black paint all over your Rembrandt and you have no damages. After all, it's still paint on canvas. The fact that other people will no longer pay as much for it or pay to see it has to be a recognizable damage.
Nonsense: spraying paint on my canvas when I don't want it there is a harm in itself. You're messing with my property. Spraying paint on my walls or my couch is a harm too, even if it has no effect (or even a positive effect) on the value of that property - as the owner, it's up to me to decide where it stays and what gets stuck to it.
If, as myrkul suggests, you simply take a photograph of the painting, then I've suffered no harm. Even if he uses that photo to make a second copy of the painting, reducing the price people will pay for mine, I've still suffered no harm for a couple reasons:
1. The money those people might have paid was all theoretical; it didn't belong to me and wasn't even promised to me, so I haven't lost it in any greater sense than I "lose" $100 million when the numbers on my PowerBall ticket don't get drawn.
2. If we consider this an actionable harm, then we must also include everything else that diminishes the resale value of my Rembrandt. Maybe a stash of new Rembrandts floods the market, or a new biography comes out that proves Rembrandt was a horrible person so no one wants his paintings, or a new biography comes out that proves
I'm a horrible person so no one wants to trade with me. All of these diminish the price I can get for my Rembrandt, so I guess we have to outlaw those as well?